Because I am involved more directly in the chemistry/formulation than Dana is, I say we do not recommend mixing different ink chemistries together. We do not know the long term consequences of joining these two chemistries together. 'Caution' implies that the ink may not be as stable as it could be. 'Risky' implies that the two chemistries could produce unwanted compounds over time. It could be totally benign to do this as a practice. It is just not a study we have undertaken chemically nor in practice.
If you asked me what my gut feeling is - I would say its probably 'ok'. Just don't hold me to that if you experience gelling, banding, or separation. Those are the three immediate problems in terms of risk. Gelling you will see in the capping station and is indicative of what may happen internally if allowed to sit. Banding will be visible in the prints but not attributable to missing nozzles. Ink separation occurs in the tank and usually results in either more density or less density of pigment being printed and usually overcome by shaking the contents together again. But over time if this occurs in the ink lines, you can have sedimentary deposits in the ink lines.